Liquefied and gaseous products are typically transported over long distances most efficiently using transmission pipelines. These pipelines are generally constructed by welding together individual pipe segments at their abutting ends from the outside as well as the inside to form a continuous pipeline. The pipeline may also need internal welding on occasions where the continuity of the pipeline has been disturbed by external forces such that adjacent pipe segments are shifted relative to each other. The welding of these pipe segments has to be performed with precision and with a minimum distortion of these segments to maintain the best possible joint strength. Accordingly, an efficient, accurate and reliable welding machine is needed to perform weldings on the pipeline joints internally.
Since the joint ends of a tubular pipeline cannot be rotated about a stationary welding source, automatic pipe welding equipment typically traverses a welding mechanism circumferentially about the abutting ends of the pipe segments. Automated machines to perform this operation are shown in U.S. Pat. No. 3,612,808 to Nelson, U.S. Pat. No. 4,525,616 to Slavens, and U.S. Pat. No. 5,059,765 to Laing. However, it is desirable to have additional precision and repeatability in the welding process than those provided by the current internal welding systems employing torch or arc welding techniques. Further, conventional automated welding equipment faces a productivity trade-off: the welding machine can travel more slowly in exchange for more reliable welding or travel faster for a higher throughput, but at a higher probability of causing welding defects. The present invention aims at eliminating the above-mentioned disadvantages and accordingly provides a welding apparatus which makes it possible to obtain a uniform weld and reduce substantially the time required for welding pipe segments together to form a pipeline.
These advantages are achieved by using as a welding agent a laser beam from a movable laser source that can be positioned inside a pipeline in the vicinity of the abutting ends of the pipe segments. The advantages of using a laser beam having a power output great enough to weld together pipe segments as a welding agent include: (1) welding in single pass for faster processing, (2) welding in a room atmosphere without special environmental preparation, (3) welding more accurately, (4) welding with little or no induced contamination, (5) welding with precisely directed and concentrated energy that results in no distortion at the end of the welding, and (6) welding with the capability of rapid starting and stopping for improved throughput.
Numerous laser welding machines have been adapted for pipeline welding. For example, U.S. Pat. No. 4,591,294, which issued on May 27, 1986 to Foulkes, discloses a pipe welding assembly in which welding of one pipe length to another is accomplished by gas lasers mounted to rotate around the axis of the pipes to weld the pipe ends together. However, the lasers disclosed in Foulkes cannot traverse the length of the pipes. U.S. Pat. No. 4,533,814, which issued on Aug. 6, 1985 to Ward, discloses a stationary laser source whose beam is directed to the pipe joint via a flexible laser beam guide. U.S. Pat. No. 4,429,211, which issued on Jan. 31, 1984 to Carstens et al., also discloses a laser welding system for welding 360.degree. around a pipe. However, the laser in Carstens is mounted remotely from the welding site and requires passive and active beam alignment systems for real time compensation of angular misalignment. U.S. Pat. No. 4,080,525, which issued on Mar. 21, 1978 to Gobetz, discloses an external welding device having laser transmitting means to orbitally direct the laser beam to the welding joints from a stationary laser source. However, a laser welding system having a laser source located remotely from the welding spot suffers from inaccuracies that result from beam misalignment, as pointed out by the laser system described in Carstens. U.S. Pat. No. 4,001,543, which issued on Jan. 4, 1977 to Bove et al., discloses a laser positioned to direct a laser beam along the axis of the pipeline and a reflection system mounted for movement through an arc of 360.degree. in the path of the laser beam to reflect that beam radially on the abutting ends of the pipeline. However, the laser disclosed in Bove is stationary and is located remotely from the actual welding locations. Hence, a need exists for a laser welder having a laser source that is located locally to the welding joint. Further, a need exists for a laser welder that can be mounted for traveling within the pipeline.